The invention relates to mass flow measurement devices and techniques for calibrating mass flow controllers.
By way of background, mass flow controllers are widely used in the semiconductor industry to provide as much precision as possible in the control of reactant gases that flow into semiconductor reaction chambers. Such mass flow controllers need to be calibrated frequently. Up to now, the best accuracy achievable in in situ calibrating of mass flow controllers is approximately one percent. There would be a large market for a more accurate system for calibrating mass flow controllers.
One prior art mass flow controller applies heat by means of an "upstream" heat coil to a portion of the gas flow diverted through a sensor tube. The flowing gas then carries heat from the upstream coil to the downstream coil surrounding the sensor tube. The resulting temperature difference is detected, and the amount of gas mass flow can be computed from the temperature difference. This prior indirect technique for measuring mass flow is undesirably influenced by thermal considerations associated with the way the mass controller is mounted in the cabinet of the semiconductor reaction chamber. It would be desirable to find a technique for mass flow measurement which is independent of such thermal considerations.
At the present time, there is no convenient, accurate way of in situ calibrating mass flow controllers using the foregoing mass flow measurement system. Presently, if there is an unknown process problem, all of the mass flow controllers often are simply replaced in a blind effort to fix the problem. Therefore, they must be disassembled and removed from the mass flow controller from the semiconductor reaction chamber to be checked and calibrated in a remote lab facility. Such disassembly and removal is highly undesirable because it generates or loosens particles that may enter the semiconductor reaction chamber, and allows airborne particles to enter the semiconductor reaction chamber. Such particles later settle on semiconductor wafers, producing defects in integrated circuits being manufactured. In situ checking of mass flow controllers allows quick identification and recalibration of ones that may have drifted. This may also provide a much-desired quick identification and correction of the process problem if it is due to a mass flow controller, without having to disassemble and replace all of the mass flow controllers before looking for other possible causes of the process problem. Therefore, it is important that mass flow controllers be checked or calibrated under the same conditions in which they will be operated, rather than removed and calibrated in a remote calibration facility.
Those skilled in the art generally believe that the weak point in most semiconductor wafer processing is inaccuracy in gas mass flow, and that future improvements in most semiconductor processes will require greater accuracy in mass flow control technology. Thus, there is a great need for a more accurate mass flow controller in the semiconductor industry.
In a gas mass flow system, if the gas flow is restricted by an aperture and if laminar flow is maintained, the amount of gas flow is proportional to the pressure drop resulting from the restriction. Then the mass flow can be directly, rather than indirectly, computed from the pressure drop. However, if the assumptions about laminar gas flow are not met, that is, if turbulent rather than laminar flow of the gases occurs, then the proportionality of mass flow to the measured pressure drop is lost, and the calculations become inaccurate. U.S. Pat. No. 3,220,256 discloses a linear flow meter including an elongated flow element. This reference discloses measurement of the difference between upstream and downstream pressures of fluid in the flow element with laminar fluid flow. An e quation is disclosed relating the pressure gradient to the volumetric flow in the elongated flow element.
There is an unmet need for a low cost, very accurate mass flow meter that maintains low fluid flow, is easily disassembled for cleaning and reassembled without changing its geometric constant, and is suitable for in situ calibration of mass flow controllers, especially in ultra-clean gas flow systems, such as semiconductor wafer processing reactors.